The present invention relates generally to liquid-atomizing spray nozzles, and more particularly to an air-assisted or “airblast” fuel nozzle for turbine combustion engines, the nozzle having a multiplicity of aerodynamic turning vanes arranged to define an outer air “swirler” providing for a more uniform atomization of the fuel flow stream.
Liquid atomizing nozzles are employed, for example, in gas turbine combustion engines and the like for injecting a metered amount of fuel from a manifold into a combustion chamber of the engine as an atomized spray of droplets for mixing with combustion air. The fuel is supplied at a relatively high pressure from the manifold into, typically, an internal swirl chamber of the nozzle which imparts a generally helical component vector to the fuel flow. The fuel flow exits the swirl chamber and is issued through a discharge orifice of the nozzle as a swirling, thin, annular sheet of fuel surrounding a central core of air. As the swirling sheet advances away from the discharge orifice, it is separated into a generally-conical spray of droplets, although in some nozzles the fuel sheet is separated without swirling.
In basic construction, fuel nozzle assemblies of the type herein involved are constructed as having an inlet fitting which is configured for attachment to the manifold of the engine, and a nozzle or tip which is disposed within the combustion chamber of the engine as having one or more discharge orifices for atomizing the fuel. A generally tubular stem or strut is provided to extend in fluid communication between the nozzle and the fitting for supporting the nozzle relative to the manifold. The stem may include one or more internal fuel conduits for supplying fuel to one or more spray orifices defined within the nozzle. A flange may be formed integrally with the stem as including a plurality of apertures for the mounting of the nozzle to the wall of the combustion chamber. Appropriate check valves and flow dividers may be incorporated within the nozzle or stem for regulating the flow of fuel through the nozzle. A heat shield assembly such as a metal sleeve, shroud, or the like additionally is included to surround the portion of the stem which is disposed within the engine casing. The shield provides a thermal barrier which insulates the fuel from carbonization or “choking,” the products of which are known to accumulate within the orifices and fuels passages of the nozzle and stem resulting in the restriction of the flow of fuel therethrough.
Fuel nozzles are designed to provide optimum fuel atomization and flow characteristics under the various operating conditions of the engine. Conventional nozzle types include simplex or single orifice, duplex or dual orifice, and variable port designs of varying complexity and performance. Representative nozzles of these types are disclosed, for example, in U.S. Pat. Nos. 3,013,732; 3,024,045; 3,029,029; 3,159,971; 3,201,050; 3,638,865; 3,675,853; 3,685,741; 3,899,884; 4,134,606; 4,258,544; 4,425,755; 4,600,151; 4,613,079; 4,701,124; 4,735,044; 4,854,127; 4,977,740; 5,062,792; 5,174,504; 5,269,468; 5,228,283; 5,423,178; 5,435,884; 5,484,107; 5,570,580; 5,615,555; 5,622,054; 5,673,552; and 5,740,967.
As issued from the nozzle orifice, the swirling fluid sheet atomizes naturally due to high velocity interaction with the ambient combustion air and to inherent instabilities in the fluid dynamics of the vortex flow. However, the above-described simplex or duplex nozzles also may be used in conjunction with a stream of high velocity and/or high pressure air, which may be swirling, applied to one or both sides of the fluid sheet. In certain applications, the air stream may improve the atomization of the fuel for improved performance. Depending upon whether the air is supplied from a source external or internal to the engine, these “air-atomizing” nozzles which employ an atomization air stream are termed “air-assisted” or “airblast.” Airblast and air-assisted nozzles have been described as having an advantage over what are termed “pressure” atomizers in that the distribution of the fluid droplets through the combustion zone is dictated by a airflow pattern which remains fairly constant over most operations conditions of the engine. Nozzles of the airblast or air-assisted type are described further in U.S. Pat. Nos. 3,474,970; 3,866,413; 3,912,164; 3,979,069; 3,980,233; 4,139,157; 4,168,803; 4,365,753; 4,941,617; 5,078,324; 5,605,287; 5,697,443; 5,761,907; and 5,782,626.
Most, if not all, of the aforementioned nozzle designs incorporate swirlers or other turning vanes to impart a generally helical motion to one or more of the fluid flow streams within the nozzle. For example, certain airblast nozzles employ an outer air swirler configured on the surface of a generally-annular member which forms the primary body of the nozzle. In this regard, the body has an inlet orifice and outlet orifice or discharge for the flow of inner air and fuel streams. A series of spaced-apart, parallel turning vanes are provided on a radial outer surface of the body as disposed circumferentially about the discharge orifice. As incorporated into the nozzle, the primary nozzle body is coaxially disposed within a surrounding, secondary nozzle body or shroud such that the radial outer surface of the primary nozzle body defines an annular conduit with a concentric inner surface of the secondary nozzle body for the flow of an outer, atomizing air stream. As each of the vanes is disposed at an angle relative to the central longitudinal axis of the swirler and the direction of air flow, a helical motion is imparted to the atomizing air which exits the nozzle as a swirling stream.
Particularly with respect to airblast or air-assisted nozzles of the type herein involved, the ability to produce a desired fuel spray which is finely atomized into droplets of uniform size is dependent upon the preparation of the atomizing air flow upstream of the atomization point. That is, excessive pressure drop or other loss of velocity in the atomization air can result in larger droplets and a coarser fuel spray. Large or non-uniform droplets also can result from a non-uniform velocity profile or other gradients such as wakes and eddies in the atomizing air flow.
Heretofore, air swirlers of the type herein involved have employed vanes of relatively simple slots or flats, or helical or curved geometries to guide and control fluid flow. In certain applications, however, slots or vanes of these types may provide less than optimum performance. In this regard, reference may be had to FIG. 1 wherein fluid flow through a pair of parallel, helical vanes is shown in schematic at 10. Each of the helical vanes, referenced at 12a and 12b, has a leading edge, 14a-b, and a trailing edge, 16a-b, respectively, and is disposed at a turning or incidence angle, θ, relative to the upstream direction of fluid flow which is indicated by arrow 18. The vanes are spaced-apart radially to define a flow passage, referenced at 20, therebetween.
As may be seen in the schematic of FIG. 1, with the fluid flow being directed to define a lower pressure or suction side, referenced at “S,” and a higher pressure or pressure side, referenced at “P,” of the vanes 12, some separation of the flow from the suction side is evident beginning at the leading edge 14 of each of the vanes. This separation, which produces the leading edge bubbles depicted by the streamlines referenced at 22a-b, and the trailing edge wakes, eddies, vorticities, or other recirculation flow depicted by the streamlines referenced at 24a-b, has the effect of reducing the area for fluid flow through the vane passages 20, and of developing strong secondary flows within the stream which can persist many vane lengths downstream of the vanes 12. Thus, and particularly for medium or high turning angles, i.e., between about greater than about 8°, a helical vane profile can result in a diminished flow volume from the nozzle, non-uniform downstream velocity profiles, and otherwise in velocity or pressure losses and than optimum performance.
Turning next to FIG. 2, the fluid flow through a pair of parallel, curved vanes is shown for purposes of comparison at 10′. As before, each of the curved vanes 12a-b′ has a leading edge 14a-b′, and a trailing edge 16a-b′, respectively, and is disposed at a turning or incidence angle, θ, relative to the direction of fluid flow which again is indicated by arrow 18. The vanes are spaced-apart radially to define a flow passage 20′ therebetween.
As compared to that of the helical vanes of FIG. 1, the flow through the curved vanes 12′ exhibits no appreciable bubble separation at the leading edges 14. However, as the trailing edges 16′ of the vanes are not parallel, that is the suction side S of vane 12a′ is not parallel to the pressure side P of vane 12b′, losses are produced and the flow becomes non-uniform at that point as shown by the separation referenced at 24a-b′. At large turning angles, i.e., greater than about 15°, the effect becomes more pronounced and may result in pressure losses, non-uniform velocity profiles, and recirculation flows downstream.
In view of the foregoing, it will be appreciated that improvements in the design of fuel nozzles for turbine combustion engines and the like would be well-received by industry. A preferred design would ensure a uniform atomization profile under a range of operating conditions of the engine.